Mechanical pulps and processes for manufacturing mechanical pulps are known. See for example U.S. Pat. Nos. 6,527,914; 6,743,332; 5,129,987; 3,388,037; 3,467,574; 3,804,944; 3,985,674; 4,534,954; 4,676,961; 4,756,799; 4,235,665; 4,136,831; 4,012,279; 3,847,363; 3,661,320; 3,873,412 and the like.
The first step in the Mechanical pulping process is the grinding or refining of wood. The Stone Ground wood (SGW) process involves making pulp by pressing logs and chips against an abrasive rotating surface. Many years ago the grinding surface used was an actual stone. In current practice specifically designed “artificial pulp stones” are available for the grinding. A Pressurized Ground Wood (PGW) process is where the grinding operation is completely pressurized.
Another type of Mechanical pulping is Refiner Mechanical Pulp (RMP) featuring atmospheric refining with no pretreatment of the wood chips. This process is one of the main mechanical pulping operations.
Thermo Mechanical Pulping (TMP) is a Mechanical pulping process that evolved from RMP and a high temperature process known as the Apslund process. Thermo Refiner Mechanical Pulping (TRMP) is a variation in Thermo Mechanical Pulping. In this case, the chips are preheated under pressure and refining is carried out at atmospheric pressure. TMP and TRMP pulps are stronger than either SCW or RMP pulps.
The third type of pulping process is a Combination of Chemical and Mechanical pulping processes. Two types of Combination processes are ChemiMechanical Pulping and SemiMechanical Pulping. There is little difference between ChemiMechanical Pulping (CMP) and SemiChemical Mechanical Pulping (SCMP). Both processes involve pretreatment of chips with chemicals, followed by mechanical refining. Four different chemical treatments are associated with these processes. These chemical treatments are: sodium hydroxide, sodium bisulfite, sodium sulfite, and acid sulfite treatment. These processes are generally used on hardwoods. Chemical treatment weakens the fiber structure allowing fibers to rupture similarly to softwood that is mechanically pulped.
ChemiThermoMechanical Pulping (CTMP) appears to be a full evolution of all Mechanical pulping methods. It includes chemical treatment at elevated temperature steaming followed by mechanical refining. This process can produce fibrous raw materials that vary considerably in properties depending upon process conditions such as wood source, sodium sulfite concentration, pH, temperature, etc.
The foregoing list is by no means exhaustive. There are innumerable combinations and variants of the pulping processes as exemplified in The Handbook of Pulping and Papermaking, 2d ed., by Christopher J. Biermann; Acronyms for mechanical Pulp: Understanding the Alphabet Soup, TAPPI Journal (December 1987), Cooper, W and Kurdin, J. A.; Leask, R. A. and Kocurek, M. J. (Editors). Mechanical Pulping (Volume 2 of Pulp and Paper Manufacture Series) Joint Textbook Committee, 1987; and Cropp, H. V., Efficient Use of Recovered Energy is a key Mechanical Pulping Goal, Pulp & Paper (April 1991), all of which are herein incorporated by reference.
In general, while having higher yields, opacities and bulk as compared to chemical pulping processes such as Kraft and Sulfite pulping processes, mechanical pulps have a relative low ISO brightness as for example a brightness of not more than 65 ISO units for hardwood mechanical pulps and a brightness of nor more than 60 ISO brightness units for softwood mechanical pulps because of the substantial amounts of retained lignin. To enhance the brightness of mechanical pulp, such pulp has been subjected to one or more subsequent bleaching stages. For example, mechanical pulp resulting from a ChemiThermoMechanical Pulping (CTMP) process can be subjected to one or more subsequent bleaching stages to form Bleached Chemical Thermomechanical Pulp (BCTMP).
Bleaching is a term applied to a semi-chemical or chemical step in a in which the mechanical pulp is treated with an active bleaching agent, such as chlorine, chlorine dioxide, ozone, oxygen, hydrogen peroxide, peroxy acids, enzymes, or a mixture thereof, at a controlled time, temperature, and pH. The desired outcome of these reactions is to brighten the mechanical pulp to ever-higher levels of brightness (the Technical Association of the Pulp & Paper Industry (“TAPPI”) or the International Organization for Standardization (“ISO”)). Brightness levels can be increased to some extent. For example, the ISO brightness of softwood mechanical pulps can be increased to about 75 ISO with one conventional peroxide bleaching stage and about 80 ISO with two conventional peroxide bleaching stages and the ISO brightness of hardwood mechanical pulps can be increased to about 80 ISO with one peroxide bleaching stage and about 85 ISO with two conventional peroxide bleaching stages. Widespread consumer preference for a brighter, whiter pulp drives manufacturers to pursue ever more aggressive bleaching strategies. However, while brightness levels are increased somewhat, other properties of the mechanical pulp are adversely impacted such as bulk and opacity which increases with increasing bleaching. While highly bleached pulps are “whiter” than their less-bleached cousins, they are still yellow-white in color. A yellow-white product is undesirable. Countless studies suggest that consumers clearly favor a blue-white over a yellow-white color. The former is perceived to be whiter, i.e., “fresh”, “new” and “clean”, while the latter is judged to be “old”, “faded”, and “dirty”.
Further optical enhancement in brightness is usually accomplished by the addition of tinting colorants, fillers, and/or Fluorescent whitening agents (FWA) or optical brightening agents (OBA) usually added during the stage during which the pulp is mixed with A optical brightening agents and processes for enhancing the brightness pulp or paper fibers are described in U.S. Pat. Nos. 5,482,514; 6,893,473; 6,723,846; 6,890,454; 6,426,382; 4,169,810; 5,902,454; and U.S. Pat. Application Publication Nos. US 2004/014910 and US 2003/0013628.